bcachefs: Make btree_iter_peek_prev() assert more precise

bcachefs: Make btree_iter_peek_prev() assert more precise

The issue this assert is guarding against is that in
BTREE_ITER_filter_snapshots mode we only want to be iterating within a
single inode number - if we iterate into another inode number with keys
for a different snapshot tree, we'll loop arbitrarily long before
finding a key we can return.

This comes up in the unit tests, where we're using inode 0 for our test
keys.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>